1. Field of the Invention
The present invention relates to a method for manufacturing an assembly of a solid polymer electrolyte membrane and electrodes and to an assembly manufactured by the method, and more particularly to a fuel cell which is made using the assembly and uses as a fuel a reducing agent such as pure hydrogen, modified hydrogen obtained from methanol or fossil fuels or methanol and as an oxidizing agent air or oxygen, and especially a fuel cell which uses a solid polymer as an electrolyte.
2. Description of the Related Art
One of the most important factors which govern the discharge performance of solid polymer type fuel cells constructed using an assembly of a solid polymer electrolyte membrane and electrodes is the reaction area at an interface of three phases formed by pores which are passages for feeding reaction gas, a solid polymer electrolyte having protonic conductivity due to containment of water, and an electrode material of electronic conductor at the interface between a solid polymer electrolyte membrane and an electrode.
Hitherto, in order to increase the three face interface, it has been attempted to apply a layer prepared by mixing and dispersing an electrode material and a solid polymer electrolyte to the interface between the membrane and a porous electrode. For example, JP-B-62-61118 and JP-B-62-61119 disclose a method which comprises coating a mixture of a solution of solid polymer electrolyte with a catalyst compound on a solid polymer membrane, hot pressing the coated membrane on an electrode material and then reducing the catalyst compound or carrying out the coating after the reduction and then carrying out the hot pressing.
JP-A-3-184266 uses a powder prepared by coating a solid polymer electrolyte on the surface of a polymer resin, and JP-A-3-295172 employs a method which comprises incorporating a powder of a solid polymer electrolyte into an electrode. JP-A-5-36418 discloses a method which comprises mixing a solid polymer electrolyte, a catalyst, a carbon powder and a fluoropolymer and forming the mixture into a film to form an electrode.
All of the above patent publications use alcohol solvents for the solutions of the solid polymer electrolyte. Furthermore, U.S. Pat. No. 5,211,984 reports a method which comprises preparing an inky dispersion comprising a solid polymer electrolyte, a catalyst and a carbon powder using glycerin or tetrabutylammonium salt as a solvent, casting the dispersion on a polytetrafluoroethylene (hereinafter referred to as "PTFE") film, and then transferring it onto the surface of a solid polymer electrolyte membrane or a method which comprises changing the exchanging group of a solid polymer electrolyte membrane to Na type, coating the above inky dispersion on the surface of the membrane, and heating and drying the coat at 125.degree. C. or higher to again change the group to H type.
However, when a catalyst-supporting carbon powder and a water repellent material such as fluoropolyer or a carbon powder subjected to water repelling treatment are simultaneously added to the solid polymer electrolyte solution, much solid polymer electrolyte is adsorbed to the water repellent material or the carbon powder subjected to water repelling treatment, and accordingly the degree of contact between the solid polymer electrolyte and the catalyst becomes insufficient and nonuniform, and no sufficient reaction area can be ensured at the interface between the electrode and the ion-exchange membrane.
Moreover, in all of the above methods, it is difficult to coat the solid polymer electrolyte at a suitable thickness on the surface of the catalyst, and in fact the thickness of coat of the polymer electrolyte cannot be controlled. Therefore, there are problems that the catalyst cannot be sufficiently coated on the polymer electrolyte, resulting in a small reaction area or thickness of the coat is too large and diffusion route of hydrogen or oxygen becomes longer to cause increase of concentration over voltage.
Furthermore, when the dispersion with an alcoholic solvent is coated on a porous substrate or when the inky dispersion is coated on a porous substrate, the dispersion cannot be directly molded on the surface of the substrate as the dispersion penetrates or permeates into the inside of the substrate and thus, complicated processing techniques such as transferring are needed.
Moreover, the above-mentioned method of directly coating the inky dispersion on the surface of the membrane requires the complicated production technique of replacing the exchange group of the membrane many times.
"Journal of Electroanalytical Chemistry", 417 (1996) 105-111 mentions that the thinner thickness of polymer electrolyte layer on the catalyst surface gives the more easy occurrence of diffusion of hydrogen and oxygen, and according to FIG. 7, when the thickness is 400 nm, the highest characteristics are obtained. However, this is a result of experimentation conducted on a smooth Pt surface, and is not concerned with the thickness of the polymer electrolyte layer on the catalyst surface in the catalyst layer of porous electode.
The inventors have disclosed a method for manufacturing electrodes using colloid of solid polymer electrolyte in JP-A-7-183035 and JP-A-8-264190. The object of the present invention is to improve these inventions and to provide an assembly of a solid polymer electrolyte membrane and electrodes having further higher performances by severely specifying colloid particle size of the solid polymer electrolyte and controlling the thickness of the coat, and to provide a solid polymer type fuel cell made using said assembly.